Water harvesting performance of biphilic surfaces

Dew collection is recognized as one of the most promising and effective methods for passive water harvesting, especially in arid and semi-arid regions where conventional water resources are limited. In this work, we conducted a comprehensive experimental investigation to evaluate how different surface wettabilities—engineered through nanostructuring and surface modification—affect the efficiency of water collection from atmospheric dew. Aluminum substrates were prepared to exhibit four distinct wetting behaviors: hydrophobic, superhydrophobic, hydrophilic, and biphilic. Each type of surface was fabricated using specific techniques tailored to modify its surface energy and micro/nano-scale topography. The superhydrophobic surface was produced by applying fluorinated silica nanoparticles coating, creating a low surface energy layer capable of repelling water droplets and minimizing droplet adhesion. In contrast, two types of hydrophilic surfaces were created: one through nano-laser surface texturing, which introduced microgrooves and increased surface roughness, and another by mechanically abrading the surface with sandpaper, enhancing its water-attracting properties through physical roughening. In addition to these single-wettability surfaces, a series of biphilic surfaces—featuring alternating superhydrophobic and hydrophilic regions—were fabricated to explore the synergistic effects of combining contrasting wettability zones. These patterns were carefully designed with varying superhydrophobic-to-hydrophilic area ratios to assess their influence on droplet dynamics and water collection rates. Among all tested configurations, the nano-laser-treated hydrophilic surface yielded the highest water harvesting performance, collecting up to 386.7 mL/m². This value represents a significant improvement—approximately 42% greater than the unpolished aluminum surface and 282% higher than the superhydrophobic surface. The enhancement in performance is attributed to the improved nucleation, spreading of droplets, and directional transport enabled by the surface's tailored nanostructures. Notably, the biphilic surfaces demonstrated non-linear trends in water harvesting efficiency depending on the area ratio of superhydrophobic to hydrophilic regions. The optimal performance was observed at a 1:4 ratio, where the synergy between rapid droplet nucleation on hydrophilic regions and efficient droplet removal via superhydrophobic pathways was maximized. Ratios beyond this threshold showed a decline in performance, suggesting a critical balance between adhesion and repellency must be maintained for optimal efficiency. The observed experimental results were interpreted based on the fundamental mechanisms of water harvesting, including nucleation of droplet, growth, coalescence, and movement. The integration of surface energy gradients and micro/nanostructuring proved to be crucial in facilitating efficient water movement and collection. These findings contribute valuable insights into the determination of functional surfaces for atmospheric water harvesting. By understanding and optimizing the interplay between surface chemistry and topography, it becomes possible to engineer materials that significantly improve passive water collection strategies in resource-scarce environments.